1. Field of Invention
The invention relates to a device for blow molding a container, as well as to the use of a tubular electric linear motor in such a device.
2. Related Art
Such blow molding devices are used, for example, in hollow stretch blow molding machines in the case of which a parison is stretch molded by the stretching rod within a blow mold in response to a thermal conditioning and is recast by the blowing pressure impact in the container. In the case of a container forming by means of the impact of blowing pressure, parisons made of a thermoplastic material, for example parisons made of PET (polyethylene terephthalate), are supplied to different processing stations within a blow molding machine.
Typically, such a blow molding machine encompasses a plurality of blowing stations, which are arranged on a common blowing wheel, and which in each case encompass a blow molding device as well as a forming tool, in which the previously tempered parison is expanded into a container by means of biaxial orientation. A heating station for preheating the parisons is thereby typically not located on the blowing wheel itself, but in a continuous annealing furnace located upstream. Typically, the expansion of the parisons takes place by means of compressed air, which is introduced into the parison, while the stretching rod is simultaneously extended for guiding the parison.
With reference to the used blowing stations, different embodiments are known. In the case of blowing stations, which are arranged on rotating transport wheels, the mold support can often be opened in a book-like manner. However, it is also possible to use mold supports, which can be displaced relative to one another or which are guided in a different manner. In the case of stationary blowing stations, which are in particular suitable to accommodate a plurality of cavities for the container molding, disks are typically used as mold supports, which are arranged parallel to one another.
The stretching rods are hereby often positioned by pneumatic cylinders and mostly in combination with a mechanical connecting rod, whereby the blow molding devices not only reach an enormous installation height, but also require a particularly stable design due to the parts, which are difficult to move as well as due to the forces appearing in the connecting rod due to the counteraction. The already high weight of such a device is thus additionally increased and the rotary motion thereof, e.g., requires much energy in a blow molding machine. Likewise, the energy consumption is very high due to the use of compressed air as drive medium. A pneumatic/mechanic positioning of the stretching rods thus likewise increases the production and operating costs of a blow molding machine, which is designed in this manner.
It is already known from German patent document DE-OS 103 25 229 to use an electric linear drive, which is constructed similarly to the functional principle of the Transrapid railroad system, for positioning a stretching rod. Such linear motors provide for a highly accurate reproducibility when carrying out stretching motions, but encompass a comparatively high construction weight and a high price.
International patent application publication no. WO 2006/108380 tries to solve this problem by combining a fluid drive and an electric linear motor and to thus combine the advantages of the respective drive systems with one another. In the context of controls, fluid drives, in particular pneumatic or hydraulic drives, make it possible to carry out positioning motions very quickly in response to high forces, which can be generated, and in response to low construction weight. However, the fluid drives encompass disadvantages with reference to the positioning accuracy, in particular when carrying out positioning motions very quickly, because control processes can only be realized to a limited extent in consideration of the system inertias. By means of combining a fluid drive with an electric linear motor, it is to now be possible to use a linear motor with relatively low output and thus also a low price and small construction weight and to only use it to correct the acceleration or braking forces, respectively, of the fluid drive. However, an important disadvantage of this combination lies in the fact that neither the weight nor the installation size of a blow molding machine can be decreased drastically, because an electric linear motor must also be integrated in addition to the fluid drive. This solution furthermore still uses a fluid drive and thus a very expensive source of energy.
European patent document EP 1 484 160 shows a drive device, which is also embodied as a linear motor. The device is connected to an actuator, which gives an actuating variable for the linear motor as a function of a comparison between a setpoint value course and an actual value, which is determined by means of measuring techniques, for a respective positioning of the stretching rod. The setpoint value course is thereby stored in the area of a setpoint value storage. By embodying the drive device as linear motor, small spatial dimensions are to provide for a high mechanical resilience and are to be used for generating large forces, which can be metered accurately. A predetermined motion profile is to be adhered to accurately by means of the control circuit and the direct evaluation of a current actual positioning of the stretching rod, which is performed by means of measuring techniques, because the linear motor can be regulated with a high dynamics and thus with only small time delays. However, the advantage is here that only a large and heavy and thus very expensive linear motor, which is dimensioned accordingly, is able to provide the forces, which are typically provided by a fluid drive.
The state of the art thus relates consistently to rotary machines, all of which are designed mechanically and/or pneumatically. Despite all of the expected advantages, the machines provided with electric drives encompass deficiencies and have consequently never been realized. A servo motor comprising spindle or belts, respectively, is thus extensive and expensive and displays considerable wear. In turn, a linear motor as in European patent document EP 1 484 160 is flat, but also expensive and requires further components, such as linear guides, etc. A linear motor as in International patent application publication no. WO 2006/108380 is only used to regulate the travel and does not provide for the required force. It is thus supported pneumatically/hydraulically, whereby all of the disadvantages of cylinders, such as high air consumption, cylinder wear, etc., reappear.